docs: no days on Instant

[iloveitaly]

Description

Add note about delta limitation of days. Not sure if this is the best place for it.

#179

Checklist

• Build runs successfully
• Type stubs updated
• Docs updated
• If docstrings were affected, check if they appear correctly in the docs as well as autocomplete

[ariebovenberg]

Thanks for posting the PR, the docs in this area can indeed be improved. I'll try and have a look soon if this is the right place. Note in any case that deltas are probably going to be changed a bit (last breaking change before 1.x 🤞 )